A black body (or blackbody) is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. A black body in thermal equilibrium (that is, at a constant temperature) emits electromagnetic radiation called black-body radiation. The radiation is emitted according to Planck's law, meaning that it has a spectrum that is determined by the temperature alone, not by the body's shape or composition. A black body in thermal equilibrium has two notable properties:                It is an ideal emitter: it emits as much or more energy at every frequency than any other body at the same temperature.        It is a diffuse emitter: the energy is radiated isotropically, independent of direction.        
An approximate realization of a black surface is a hole in the wall of a large enclosure. Any light entering the hole is reflected indefinitely or absorbed inside and is unlikely to re-emerge, making the hole a nearly perfect absorber. The radiation confined in such an enclosure may or may not be in thermal equilibrium, depending upon the nature of the walls and the other contents of the enclosure.
Real materials emit energy at a fraction—called the emissivity—of black-body energy levels. By definition, a black body in thermal equilibrium has an emissivity of ϵ=1.0. A source with lower emissivity independent of frequency often is referred to as a gray body. Construction of black bodies with emissivity as close to one (1) as possible remains a topic of current interest. A white body is one with a rough surface that reflects all incident rays completely and uniformly in all directions.
In astronomy, the radiation from stars and planets is sometimes characterized in terms of an effective temperature, the temperature of a black body that would emit the same total flux of electromagnetic energy.
Extended Area Blackbodies
Extended area blackbodies are commonly used for testing infrared cameras and other thermal detection devices. Blackbodies are used as thermal sources to provide a desired radiance or apparent temperature to a device under test or to illuminate a target with a known radiance or apparent temperature or to provide a desired temperature difference between the features in a target.
For an ideal blackbody with an emissivity of one (1), the only parameter required to calculate the radiance of the source is the temperature of the blackbody. For a system with non-unity emissivity, other factors must be considered in order to produce a desired radiance, including the emissivity and the radiance of the environment onto the surface of the source. Another factor to be considered is that the temperature of the surface of the source is needed, but the temperature may not be measured on the blackbody surface. Also, for extended area blackbodies, the temperature is typically measured in only one place, therefore the uniformity of the surface temperature must also be considered. An ideal blackbody would have a unity emissivity, and the same temperature across the entire surface as that of the point where the temperature is measured.
Extended area blackbodies are commonly used for testing infrared cameras and other thermal detection devices that have a field of view and require large uniform scenes or targets with a large extent. Cavity blackbodies or point sources rely mainly on their geometry (spherical, conical, reverse cone, tube) to create emissivity close to 1.0. These type of blackbodies are limited in size, are slow and do not have the uniformity and precise control required to test imaging systems. Extended area blackbodies provide the size and performance needed, but lack any geometric properties that enhance emissivity and rely on surface coatings to achieve high emissivity.